The invention relates to an electric switch, in particular a load switch or power circuit breaker for low-voltage applications, comprising a housing, at least one pair of contacts consisting of a fixed and a movable contact which can assume a first and second position relative to the fixed contact; a spring system having a rigid arm and a leaf spring, which leaf spring is hinged by one end to the arm, in such a manner that an arm-leaf spring assembly in the form of a toggle mechanism with a first and second end is formed, in which the first end is hinged to a movably supported element which bears the movable contact, and in which the second end is disposed hingedly supported at a distance from and opposite the fixed contact; control means acting upon the arm-leaf spring assembly for taking the contacts into the first and/or second position, in which the leaf spring is released or tensioned; and latching means for locking the contacts in the first and/or second position.
A switch of this type, in which the movable contact is disposed in a linear, slidable manner by means of the movably supported element, is known from European Patent Application 0,322,986 A1.
Switching components for low-voltage applications must meet a number of requirements, inter alia with regard to the contact distance, i.e. the distance between the contacts in the open (non-conducting) state of the switch; the contact force, i.e. the force with which the contacts are held against each other in the closed (conducting) state of the switch; the contact opening speed; the short-circuiting capacity, i.e. the maximum permissible short-circuit current under the influence of which the contacts do not open by themselves; the contact wear, also called contact erosion; and if desired they must have provisions enabling them to separate from each other contacts which have become welded together.
The abovementioned requirements are for low-voltage applications, i.e. direct or alternating voltages in the range of about 42-750 V, depending on the nominal amperage of the switch, i.e. the maximum current intensity which the switch must be able to conduct continuously. Standardised amperages are specified in practice.
As in the case of electronic components, efforts are currently being made to achieve the smallest possible dimensions also in electrical components for low-voltage use. The reason for this is to accommodate as many components as possible in installation boxes or wiring boxes of standard dimensions, in order to improve the space filling factor, or to be able to utilise installation boxes or apparatus housings of reduced dimensions with the same number of components.
An obvious solution for reducing the switch length by reducing the length of the arm-leaf spring system in its extended state does little or nothing to help.
Shortening the leaf spring is possible only to a very limited degree, on account of the inadmissible increasing mechanical stress in the leaf spring material when the length is reduced. The selection of weaker spring material is limited, inter alia, because of the requirements concerning the contact force and the contact opening speed.
Shortening the arm of the arm-leaf spring assembly, if the arm is supported with one end rotatable in the housing and the leaf spring is coupled to the movable element, is also possible only to a limited degree because of the requirements concerning the contact distance. For, in order to achieve a desired contact distance in the case of an arm of reduced dimensions, this shorter arm must be turned from the extended position of the arm-leaf spring assembly through a greater angle than a longer arm. This greater angular rotation, in particular the time required for it, has an adverse effect on the contact opening speed of the switch.
Although in the opposite case, i.e. when the leaf spring is firmly clamped and the arm is coupled to the movable element, shortening of the arm does not have any direct influence on the contact opening speed, the greater angular rotation of the arm, which also occurs in that case, results in an undesirable greater friction force on the guiding of the movable element. Apart from the disadvantages of greater wear and mechanically narrower tolerances in the guiding of the slidable element, the greater friction force also has an adverse effect on the speed of movement of the slidable element.
A desired, effective reduction of the switch length can consequently not be achieved only by simply changing the dimensions of the arm-leaf spring assembly.
French Patent Application 2,061,682 discloses an electric switch comprising a toggle mechanism, spring means, a latching mechanism and control means. The toggle mechanism is made up of hingedly coupled arms, whereas the spring means comprise conventional compression and draw springs. The latching mechanism is designed in order to achieve a small force exerted by the toggle mechanism in its extended or nearly extended position on the control means. With regard to obtain an effective reduction of the switch length, while maintaining the requirements for a desired nominal amperage, switching speed, contact force, contact distance and acceptable mechanical loading, this switch construction offers no alternatives.